This is true. But your ig will be on a shelf up high. So as I read your habitat description, your ig will need a four ft. linear heated area. to get that heated to proper hot side temps, as Brian said, you will need to use spot heaters in a line to heat the whole thing. In addition to the UVB tube and row of good lamps for lighting, you may need to use some CHE's (ceramic heat emitters-find them in on-line pet stores) They last for more than a decade, so I see CHE's as a one-time investment. igs don't like extremely bright (harsh) lighting, so the distance between your ig and the surface of the light bulbs should be adjusted to soften the bright white light a little.
It might be best to use LED bulbs for light. That would eliminate the overheating problem, while the CHE's control most of the heat, day and night. I strongly urge you to get a thermostat that uses a remote probe to sense the heat where you want it. Zoomed sells one, but Spyder Robotics makes high quality solid state thermostats that last for more than 20 years. Thermostats are adjustable. Then when the lights turn off at night, the thermostat can control the temperature with the CHE's, and it will be truly dark, which iguanas need at night.
BTW, iguanas on narrow shelves can't turn around. If a shelf is wider but still too narrow, igs break their tails when they make the sharp turn. I'd say the shelf should be 12 to 14 inches wide. Also Igs love to lay on soft pillows. In the wild they lay on each other in small groups.